Re: [PATCH net v3 2/2] net: stmmac: preserve real_num_tx_queues on mqprio setup failure

With the FPE preemption-class mapping error now propagated from
stmmac_fpe_map_preemption_class(), tc_setup_dwmac510_mqprio() can fail
on the mapping step. The error path used to call stmmac_reset_tc_mqprio(),
which resets the number of real TX queues to priv->plat->tx_queues_to_use
(the platform maximum), overwriting the value that was active before the
offload was attempted (for example a lower count left over from a previous
mqprio configuration).

The issue can be triggered using the following configuration:

  # First mqprio config lowers the hw queue count below the platform
  # default (e.g. 8 TX queues).
  $tc qdisc add dev eth0 root handle 1: mqprio queues 2@0 2@2

  # Replace mqprio configuration with a second one that fails FPE
  # preemption-class mapping. stmmac driver resets the real_num_tx_queues
  # to the platform maximum, losing the previous configuration.
  $tc qdisc replace dev eth0 root handle 2: mqprio queues 2@0 2@2 fp E P

Save ndev->real_num_tx_queues before lowering it and restore it,
together with the TC-to-queue and priority-to-TC mappings, when the FPE
preemption-class mapping fails, instead of resetting the queue count to
the platform maximum.

Note that a failed setup makes the qdisc layer run mqprio_destroy() on
the new qdisc. Because priv->hw_offload is only assigned after
ndo_setup_tc() succeeds, mqprio_destroy() calls netdev_set_num_tc(dev, 0),
so dev->num_tc ends up 0 regardless of the driver-side restore and the
previous qdisc is not reactivated. The restore is still needed to keep
real_num_tx_queues and to avoid leaving the failed configuration's
TC-to-queue and priority-to-TC mappings in place.

Fixes: 195e4f409a40 ("net: stmmac: support fp parameter of tc-mqprio")
Signed-off-by: Lorenzo Bianconi <redacted>
---
 drivers/net/ethernet/stmicro/stmmac/stmmac_tc.c | 82 +++++++++++++++++++------
 1 file changed, 62 insertions(+), 20 deletions(-)

diff --git a/drivers/net/ethernet/stmicro/stmmac/stmmac_tc.c b/drivers/net/ethernet/stmicro/stmmac/stmmac_tc.c

-	err = netif_set_real_num_tx_queues(ndev, num_stack_tx_queues);
+	err = stmmac_set_ndev_tcs(ndev, qopt->num_tc, tc_to_txq);
+	if (err)
+		goto error_reset_tc;
+
+	ndev_num_tx_queues = ndev->real_num_tx_queues;
+	err = netif_set_real_num_tx_queues(ndev, num_tx_queues);
 	if (err)
-		goto err_reset_tc;
+		goto error_reset_tc;
 
 	err = stmmac_fpe_map_preemption_class(priv, ndev, extack,
 					      mqprio->preemptible_tcs);
 	if (err)
-		goto err_reset_tc;
+		goto error_reset_num_tx_queues;
 
 	return 0;
 
-err_reset_tc:
-	stmmac_reset_tc_mqprio(ndev, extack);
+error_reset_num_tx_queues:
+	if (netif_set_real_num_tx_queues(ndev, ndev_num_tx_queues))
+		netdev_warn(ndev, "Failed to restore %u TX queues\n",
+			    ndev_num_tx_queues);
+error_reset_tc:
+	stmmac_set_ndev_tcs(ndev, ndev_ntc, ndev_tc_to_txq);
+	for (i = 0; i < ARRAY_SIZE(ndev_prio_tc_map); i++)
+		netdev_set_prio_tc_map(ndev, i, ndev_prio_tc_map[i]);